|Name(s):||UTAU, Vocal Synthesizer Tool UTAU|
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UTAU, also known as Vocaloid Synthesizer Tool UTAU, is a freeware vocal synthesizer program that runs on Windows and allows users to assemble their own vocal samples into a voicebank.  It was created in 2008 by an individual who goes by both Ameya and Ayame (飴屋 and 菖蒲).  Voicebanks created with this program are known as UTAUloids, as a reference to the commercially-made Vocaloid software, and many UTAUloids are accompanied by anime mascots meant to mimic those of Vocaloids 
UTAUloid fandom is diverse, with some people involved specifically in order to make and share voicebanks and music, while others are more interested in creating fanart and FPF involving various UTAUloid mascots.
Because the UTAU program is designed to allow users to make their own voicebanks, there are dozens of UTAUloids available online. Moreover, many UTAUloids fail to become popular, and some are even discontinued or deleted by their creators.  The UTAUloid mascots frequently featured in fanworks represent a relatively small slice of the voicebanks available.
UTAU fandom is sometimes considered an offshoot of Vocaloid fandom, and many people are involved in both fandoms. Because of the heavy overlap between the two fandoms, many UTAU-focused works also include Vocaloid characters, or characters associated with other vocal synth programs such as CeVIO and SynthV. On Fanfiction.net, there is no separate category for UTAUloid works, and these works can instead be found by searching the Vocaloid section.
The first recorded UTAU voicebank, Kasane Teto, was conceived as an April Fools joke by members of the Vip News Bulletin@2ch forum. She was created by TWINDRILL using vocal samples from Oyamano Mayo and released as a "Vocaloid" on April 1st, 2008. In order to mimic Vocaloids, the Kasane Teto voicebank was represented by an anime mascot with drill-shaped pigtails wearing an outfit similar to Hatsune Miku's. However, in contrast to Miku's marketable teenage persona, Teto was described as being 31 years old and her gender was given as "chimera." To the surprise of her creators, Teto rapidly gained popularity, even after her status as a UTAUloid was revealed. She went on to be featured in songs by Vocaloid producers such as Lamaze-P and in Crypton Future Media's Project DIVA game series 
Kasane Teto's success inspired a series of joke Vocaloids created and released by members of Vip News Bulletin@2ch, including Namine Ritsu and Tsukone Tei. An UTAUloid named Momone Momo was released in May 2008, using vocal samples from Fujimoto Momoko. She was often grouped together with Teto and the default UTAU voicebank (usually nicknamed Utane Uta or Defoko), with the three being referred to as the "Three UTAU Girls."
As of March 2022, the most popular pairings involving UTAUloids on AO3 are:
- Kagamine Len/Sukone Tei
- Hatsune Miku/Kasane Teto
- Namine Ritsu/Yokune Ruko
Songs created with UTAU
- Triple Baka by Lamaze-P feat. Hatsune Miku, Kasane Teto, and Akita Neru
- Boom Boom Boom by KIRA feat. GUMI English, Ham, and others
- Error by niki, covered by kyaami with Namine Ritsu
- Darling by MARETU, covered by akem with Darling Prism
- Vocal Vision by AnarchySong
- Miku & Friends™ in: disaster house of friendship by charcolaughs (charcolor)
- UTAU download(Japanese)
- UTAU download (English)
- UTAU User Mutual Aid Society @wiki(Japanese)
- UTAU Wiki
- UTAU tag on NicoNicoDouga
- UTAU tag on pixiv
- UTAU tag on DeviantArt
- UTAU fanfiction on AO3